DESCRIPTION (appended verbatim from investigator's abstract): Apoptosis is a program of cellular suicide initiated by a wide range of physiological stimuli. Acting in a cellautonomous manner the apoptotic process eliminates superfluous or harmful cells without damaging neighboring cells. The signaling events which regulate the decision to die are only beginning to be understood. The goal of this proposal is to elucidate a signaling pathway responsive to the Drosophila Reaper protein a central regulator of apoptosis. We and others have shown that appropriate extracts prepared from Xenopus eggs faithfully recapitulate both biochemical and morphological events of apoptosis in vitro. Using this system we discovered that Drosophila Reaper protein can induce apoptosis in a vertebrate context (subsequently confirmed using mammalian cells). However it remains unclear whether vertebrate cells contain Reaperlike protein or perhaps activate this pathway through other means. Reaper triggers release of cytochrome c from the mitochondria to the cytosol promoting activation of caspases and ensuing cell death. We identified a novel Reaperbinding protein Scythe which is essential for Reaper action. The Specific Aims of this proposal focus on elucidating the molecular mechanism whereby Reaper/Scythe triggers cytochrome c release and on the identification of vertebrate regulators of Scythe.